1. Field of the Invention
This invention relates to the remote measurement of relative bearing and tilt in hostile environments such as occurs in oil and gas wells and the like. More specifically, the invention relates to a method and apparatus for generating electrical signals characteristic of relative bearing and the sum of relative bearing and tilt in a manner such that these electrical signals can be reliably transmitted to locations external to the bearing and tilt measurement apparatus.
2. Description of the Prior Art
During the course of drilling a well, making various well log measurements and excursions in a well, or while performing workover procedures, it is customary and frequently necessary to know the actual path the well bore takes relative to the wellhead location. Technical literature suggests, records, and reports a vast variety of methods, downhole apparatus, and related equipment for estimating, predicting, and making direct as well as indirect well bore path measurements. Such suggested methods have been based on a variety of scientific principles and theories, including gravitational, sonic and seismic techniques, as well as electrical measurements, optical systems, drilling parameter monitoring schemes, and various combinations of same. One of the more reliable and most practical techniques for determining a well bore path in the earth's substrate is the use of a downhole instrument referred to as an inclinometer. An inclinometer is employed to continuously measure the bearing and tilt of a borehole as a function of the inclinometer's distance from the wellhead. By using the recorded inclinometer data, the well bore path can be mathematically computed. The tilt, as the word implies, is quantitatively the number of degrees that the well bore deviates from true vertical (i.e., an imaginary line passing through the earth's center to the well opening at the earth's surface). A zero degree tilt corresponds to a perfectly vertical wellbore path (i.e., straight down), whereas a ninety degree tilt means that the well bore is disposed parallel to the earth's surface (i.e., horizontal). The bearing is an azimuth measurement corresponding to bird's-eye view looking straight down toward the center of the earth. Bearing ranges from 0 to 360 degrees and is a relative measurement necessitating an absolute reference direction such as true or magnetic north; hence, the use of the phrase "relative bearing". Inclinometers have been used by themselves as well as in combination with a variety of other downhole tools. In practice, an inclinometer is usually positioned in a tool or carriage that aligns the inclinometer such that its axis is substantially collinear with the well bore axis. The instrument's response to its own relative bearing and tilt then directly corresponds to the well bore's bearing and tilt at any given downhole location.
The scientific principles which literature suggests as a basis for making such tilt and relative bearing measurements are almost as numerous as the principles suggested for determining well bore direction. For purposes of this invention, the subject matter deals with the response of at least one pendulum mass for determining tilt and relative bearing. Since the 1950's, inclinometers employing pendulum masses have been commercially used and represent one of the more reliable types of systems. Yet they still have limitations in terms of reliability as well as durability and long term reproducible linearity which limits commercial success and invites improvement.